Medical imaging is one of the most useful diagnostic tools available in modern medicine. Medical imaging allows medical personnel to non-intrusively look into a living body in order to detect and assess many types of injuries, diseases, conditions, etc. Medical imaging allows doctors and technicians to more easily and correctly make a diagnosis, decide on a treatment, prescribe medication, perform surgery or other treatments, etc.
There are medical imaging processes of many types and for many different purposes, situations, or uses. They commonly share the ability to create an image of a region of the body of a patient, and can do so non-invasively. Examples of some common medical imaging types are nuclear medical (NM) imaging such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), electron-beam X-ray computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). Using these or other imaging types and associated machines, an image or series of images may be captured. Other devices may then be used to process the image in some fashion. Finally, a doctor or technician may read the image in order to provide a diagnosis.
In traditional PET imaging, a patient is injected with a radioactive substance with a short decay time. As the substance undergoes positron emission decay, it emits positrons which, when they collide with electrons in the patient's tissue emit two simultaneous gamma rays. The gamma rays emerge from the patient's body at substantially opposite directions. These rays eventually reach a scintillation device positioned around the patient. There is often a ring of scintillation devices surrounding the patient. When the gamma rays interact with oppositely positioned scintillation devices, light is emitted and detected. The light is usually transmitted through a lightguide to a photodetector. The light detected by the photodetector is then interpreted by a processor to enable an image of a slice of the region of interest to be reconstructed.
In PET (as well as SPECT) it is important to match the scintillator emission wavelength to the photodetector's optimal wavelength quantum efficiency (QE). For example, a typical photomultiplier tube (PMT) used in PET applications has a peak wavelength sensitivity at 420 nm while a typical LSO scintillator used in PET emits at 420 nm. Therefore, PMTs and LSO are very well matched in terms of wavelength matching. LSO is a very good scintillator for a PMT and is reasonably matched also for other silicon-based photodetectors such as avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs). Scintillators for PET may be made from crystal materials such as, but not limited to, LSO, YSO, LYSO, LuAP (i.e., LuAlO3:Ce), LuYAP, or LaBr3.
The phoswich approach has been used to improve the detection in PET applications by determining the depth-of-interaction (DOI) in the detector. PET scanners are typically made of long, thin detectors with high stopping power to meet high sensitivity requirements. In the absence of DOI information, however, the thickness of the scintillator reduces the spatial resolution due to parallax error. To compensate for reduced spatial resolution, detectors with DOI capability have been used. DOI capability can determine the location of the gamma interaction in the direction of the incident gamma (i.e., depth from the surface of the detector).
One way to implement DOI capability is to use a multi-layer detector, in which the layers are made of material with different scintillation properties. Because the layers have different characteristics, when a gamma event is detected it is possible to identify which layer absorbed the gamma photon and so to determine more accurately the spatial interaction location in three dimensions.
A conventional “phoswich” thus is a detector with two or more layers of different scintillators. Phoswich detectors comprising two or more scintillator layers offer a means to simultaneously achieve both high sensitivity and high spatial resolution in nuclear imaging. Each scintillator layer typically has a distinct decay time that allows the DOI of a gamma ray to be determined via pulse shape determination techniques. That is, layer identification is done by using differences in scintillation decay time inherent in the scintillators and pulse shape discrimination techniques.
The use of different types of scintillators in a phoswich may result in different light yields, emission spectra, densities, effective atomic numbers, and indices of refraction, which can often result in compromises in performance of the phoswich.